1. Field of the Invention
The present invention relates in general to systems for diagnosing a catalytic converter arranged in an exhaust system of an internal combustion engine, and more particularly to systems for diagnosing the deterioration of the catalyst of a three-way type catalytic converter in the exhaust system of the internal combustion engine by using output signals issued from respective air-fuel ratio sensors arranged upstream and downstream of the converter.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional catalyst deterioration diagnosing system of the above-mentioned type will be outlined prior to making a detailed explanation of the present invention, which conventional system is shown in, for example, Japanese Patent First Provisional Publications Nos. 63-205441 and 3-57862.
That is, in such publications, there is described a so-called "air-fuel ratio feedback control system" in which two air-fuel ratio sensors (viz., oxygen sensors) are arranged upstream and downstream of a three-way catalytic converter in an exhaust system of an internal combustion engine, and in which the air-fuel ratio feedback control is carried out by using basically the output signal from the upstream sensor. The output signal from the downstream sensor is used for correcting the feedback control in a so-called "learning control" fashion.
The air-fuel ratio control system has further a catalyst deterioration diagnosing system incorporated therewith, which carries out a catalyst deterioration diagnosis by using or comparing the output signals from both the upstream and downstream sensors.
The diagnosing system is provided by practically embodying the fact that the deterioration degree of the catalyst (viz., catalytic converter rhodium) and the oxygen storage ability of the same have a certain correlation with each other.
During execution of the air-fuel feedback control, the amount of fuel fed to the engine is controlled in a proportional-plus-integral controlling fashion substantially based on the output signal from the upstream oxygen sensor. The upstream sensor thus issues such an output signal as shown in FIG. 11(a) in which richer and leaner conditions repeat periodically. While, the downstream sensor issues such an output signal as shown in FIG. 11(b) in which the amplitude of the signal is very small and the periodic cycle of the same is very elongated as compared with that of the upstream sensor. This is because the fluctuation of oxygen concentration remained in the gas exhausted from the converter is greatly moderated by the oxygen storage ability of the catalyst.
When, due to a prolonged usage or so, the catalyst of the three-way catalytic converter becomes poor in the catalytic activity, the oxygen storage ability of the same is lowered. Under this condition, the oxygen concentration in the exhaust gas upstream of the converter and that in the exhaust gas downstream of the converter fail to show a satisfactorily marked difference. That is, under such condition, as is seen from FIG. 11(c), the downstream sensor is forced to issue an output signal which is similar to that (viz., the signal of FIG. 11(a)) from the upstream sensor.
In the conventional deterioration diagnosing system disclosed in the above-mentioned publications, the ratio (viz., T1/T2) between the rich/lean reverse period "T1" of the signal from the upstream oxygen sensor and that "T2" of the signal from the downstream oxygen sensor is derived, and when the ratio "T1/T2" exceeds a predetermined value, it is judged that the catalyst in the converter has been severely deteriorated. However, due to the nature of the system, usage of the rich/lean reverse period "T2" of the output signal from the downstream sensor as a factor for judging the catalyst deterioration fails to evaluate the catalyst deterioration in a quantitative manner. That is, in the conventional diagnosing system, the criterion for the judgement of the severe deterioration of catalyst can not be set up at will. This will be understood from the graph of FIG. 12 which shows a relationship between the oxygen storage ability of the converter and the ratio (viz., T1/T2) of rich/lean reverse period between the output signals from the upstream and downstream sensors. When the catalyst is deteriorated to a certain degree, the rich/lean reverse of the output signal from the downstream sensor becomes generally synchronized with the rich/lean reverse of the output signal from the upstream sensor. Thus, under this condition, even in both regions having a deterioration smaller and greater than the certain degree, more detailed diagnosis for the catalyst deterioration is not expected. This tends to induce an undesirable condition wherein the severe deterioration may be judged even when a significant catalytic activity is still remained in the catalyst.